Method and apparatus of controlling oxygen supply in oxyfuel combustion boiler

An apparatus of controlling oxygen supply in an oxyfuel combustion boiler having a primary recirculation line for introduction of a portion of an exhaust gas taken out by a recirculation branch line to a mill as primary recirculated exhaust gas and for supply of pulverized coal pulverized by the mill to a burner of the boiler using the primary recirculated exhaust gas, a secondary recirculation line for supply of another portion of the exhaust gas recirculated to a wind box of the boiler as secondary recirculated exhaust gas, an OAP recirculation line for supply of the rest of the exhaust gas recirculated to an OAP as OAP recirculation exhaust gas, an air separation unit, a primary oxygen mixing line for supply of a portion of oxygen produced by the air separation unit to said primary recirculation line as primary oxygen, a secondary oxygen mixing line for supply of another portion of the oxygen to the secondary recirculation line as secondary oxygen, an OAP oxygen mixing line for supply of yet another portion of the oxygen to the OAP recirculation line as OAP supplied oxygen, a direct oxygen feed line for supply of the rest of the oxygen directly to the burner as directly supplied oxygen, a total gas amount regulator provided for the recirculation branch line, a primary oxygen regulator provided for the primary oxygen mixing line, a secondary oxygen regulator provided for the secondary oxygen mixing line, an OAP oxygen regulator provided for the OAP oxygen mixing line, a direct oxygen regulator provided for the direct oxygen feed line, a coal measuring device for measuring at least one of a fuel ratio and a carbon content of coal, an unburned combustible measuring device for measurement of unburned combustibles in the exhaust gas using ash obtained by dedusting and a NOx density monitor for measurement of a NOx density in the exhaust gas at a boiler outlet, said apparatus comprising:

• a controller inputted with predetermined relationships, the relationships being determined by
• measuring at least one of the fuel ratio and the carbon content of coal by the coal measuring device,
• obtaining by calculation, upon stable combustion of the coal in the boiler, a point P1 representing a relationship between at least one of the fuel ratio and a boiler-brought-in oxygen density or the carbon content of coal and the boiler-brought-in oxygen density,
• executing the same operation as the operation to obtain the point P1 for each of one or more kinds of coal thereafter to obtain one or more points P2, and obtaining from a line connecting the points P1 and P2 a first relationship between at least one of the fuel ratio and the boiler-brought-in oxygen density or the carbon content of coal and the boiler-brought-in oxygen density,
• obtaining by calculation, upon stable combustion of the coal with at least one of the fuel ratio and the carbon content of coal having been measured in the boiler, a further point P1 representing a relationship between at least one of the fuel ratio and a rate of the directly supplied oxygen to the burner to a total oxygen amount from the air separation unit or the carbon content of coal and the rate of directly supplied oxygen to the burner to the total oxygen amount from the air separation unit, and
• executing the same operation as the operation to obtain the further point P1 for each of one or more kinds of coal thereafter to obtain one or more further points P2, and obtaining from a line connecting the further points P1 and P2 a second relationship between at least one of the fuel ratio and the rate of the directly supplied oxygen or the carbon content of coal and the rate of the directly supplied oxygen to the total oxygen amount,
• wherein said controller, inputted with the predetermined relationships, is configured to control the oxygen supply in the oxyfuel combustion boiler based on at least one of the fuel ratio and the carbon content of the coal,
• wherein the controller is configured to perform reducing a total amount of gases recirculated to increase the boiler-brought-in oxygen density, or increasing the rate of directly supplied oxygen to the total oxygen amount in case that at least one of the fuel ratio and the carbon content of the coal is higher than a predetermined value, and
• wherein the controller is configured to perform increasing the total amount of gases recirculated to reduce the boiler-brought-in oxygen density or reducing the rate of the directly supplied oxygen to the total oxygen amount in case that at least one of the fuel ratio and the carbon content of coal is lower than the predetermined value.